Any backup solution requires
the simultaneous failure of two separate media in order to incur data
loss. My own experience indicates that the chance of hard drive failure
is significantly smaller than the failure of backup medias such as tapes
or CD-Rs. Furthermore,
reinstalling a package-based Unix system, such as GNU/Linux Debian from
scratch, is comparable to restoring it from a backup, given that all
configuration info can be quickly restored.
While the cost of hard drives
don't scale very well beyond the disk limit of a normal PC (200-400
GBytes), most power users and small companies will have a backup volume
only a fraction of this limit. Within this limit, the cost per Mbyte
is similar. Therefore, as far as user data goes, and particularly on
Linux systems, backing up your user data and configuration info to another
hard drive seems to make sense.